U.S. Pat. No. 4,386,112 to Eaton et al. describes a method and apparatus for applying an abrasive layer onto the surface of a metal gas turbine engine blade. The layer comprises silicon carbide (SiC) particles in a metal matrix which contains nickel, about 15-22% chromium and about 4-8% aluminum (NiCrAl). The layer is applied by injecting SiC particles and NiCrAl powder particles into a conventional plasma spray stream. The SiC particles are injected into the plasma stream at a location axially downstream of the location at which NiCrAl powder particles are injected into the stream. As is typical of plasma spray processes, the plasma stream heats the particles, and impacts them upon a substrate, thereby forming an abrasive coating layer on the substrate.
Eaton et al. stresses that the injection location of the ceramic and metal powder particles into the spray stream is of critical importance in the fabrication of the abrasive coating layer. According to Eaton et al., the residence time of the ceramic particles stream must be short, otherwise the angular cutting edges on the ceramic particles will be destroyed. Furthermore, the ceramic particles must be injected into the spray stream at a location which is close to the substrate, in order to minimize acceleration of the ceramic particles by the stream. If the velocity of the ceramic particles is too high, the particles bounce off the substrate as they strike it. In the examples discussed in Eaton et al., the axial distance between the substrate and the location at which the ceramic particles are injected into the spray stream is about 1.5 millimeters (mm), while the distance between the substrate and the location at which the metal particles are injected into the spray stream is about 60 mm.
The coating layer formed using the procedures by Eaton et al. comprises a plurality of individual layers of the ceramic and metal; the metal acts as a matrix which secures the ceramic particles within the layer. Each layer of metal is separated from adjacent layers by oxide films. Such films are a direct result of the spray application of the NiCrAl alloy powder using conventional plasma spray techniques, i.e., in an air atmosphere.
Coatings produced in accordance with Eaton et al. have a maximum use temperature of about 540.degree. C. Above such temperatures, the matrix material undergoes substantial oxidation, and mechanical properties such as creep strength are degraded; furthermore, a solid state reaction takes place between the matrix and the SiC particles which significantly degrades the properties of the layer.
Notwithstanding the benefits associated with the development of coatings such as those described by Eaton et al., they have significant limitations; for example, they are not suited for use in the turbine section of an aircraft gas turbine engine. Accordingly, the industry continues to search for a combination of matrix material and abrasive material which have substantially better properties than those described by Eaton et al. Such properties include oxidation resistance, high temperature strength, high temperature abrasiveness, and chemical stability.